This invention relates generally to a method of treating a surface with a solution that enhances the solderability of the surface and, more importantly, acts as an anti-tarnish or anti-corrosion agent for the surface. The method is particularly useful in the fabrication and assembly of printed circuit boards.
Soldering is generally used for making mechanical, electromechanical, or electronic connections to a variety of articles. The distinction between expected functions of the joints is important because each application has its own specific requirements for surface preparation. Of the three soldering applications, making electronic connections is the most demanding.
In the manufacture of electronic equipment utilizing printed circuits, connections of electronic components to the printed circuits are made by soldering of the leads of the components to the through-holes, surrounding pads, lands and other points of connection (collectively, xe2x80x9cAreas of Connectionxe2x80x9d). Typically the connection occurs by wave soldering techniques.
To facilitate this soldering operation, the printed circuit fabricator is required to arrange that the through-holes, pads, lands and other points of connection are receptive to the subsequent soldering processes. Thus these surfaces must be readily wettable by the solder and permit an integral conductive connection with the leads or surfaces of the electronic components. Because of these needs, printed circuit fabricators have devised various methods of preserving and enhancing the solderability of surfaces.
One means of arranging good solderability of the surfaces in question is to provide the surfaces with a pre-coating of solder. This is typically performed by a process called hot air solder leveling or through some type of plating process. In printed circuit fabrication, however, this method has several drawbacks. The use of hot air solder leveling may cause unacceptably high rate of defects due to shorts, particularly when dealing with small circuits. If plating is used, since it is not easy to selectively provide these areas with solder, all conductive areas of the board must be solder plated causing severe problems with the subsequent application of solder mask. In addition, the foregoing processes are inefficient and relatively expensive.
Another means of arranging good solderability of these surfaces is to plate them with a final finish coating of a precious metal such as gold, palladium or rhodium. U.S. Pat. No. 5,235,139 (Bengston, et. al.), the teachings of which are incorporated herein by reference, proposes a method for achieving this previous metal final finish. Bengston et. al. propose plating the copper areas to be soldered with electroless nickel-boron, followed by a precious metal coating such as gold. See also U.S. Pat. No. 4,940,181 to Juskey, Jr. et al., the teachings of which are incorporated herein by reference for a similar process which teaches the plating of electroless copper, followed by electrolytic copper, followed by nickel followed by gold as a solderable surface. These processes work well but are time consuming and expensive.
Various attempts have been made to selectively apply solder to the necessary areas only. One such method, described in U.S. Pat. No. 4,978,423 to Durnwith et. al., the teachings of which are herein incorporated by reference, involves use of organic etch resists over the solder plated areas of connection followed by selective stripping of tin-lead from the copper traces before application of the solder mask. See also U.S. Pat. No. 5,160,579 to Larson, the teachings of which are incorporated herein by reference, for other known selective solder processes.
Soldering directly to copper surfaces has been difficult and inconsistent. These problems are due mainly to the inability of keeping the copper surfaces clean and free of oxidation throughout the soldering operation. Various organic treatments have been developed to preserve copper surfaces in a readily solderable state. For example, see U.S. Pat. No. 5,173,130 (Kinoshita) which teaches the use of certain 2-alkylbenzimidazoles as copper pre-fluxes to preserve the solderability of the copper surfaces. Treatments such as those taught in Kinoshita have proven successful, but there is still need to improve the reliability of the process.
The method of preserving solderability proposed herein is the coating of copper surfaces to be soldered with an immersion or electroless silver plate prior to soldering. It has been found, however, that when the foregoing method is used, the silver coating has a tendency to develop outgrowths or filaments via an electromigration mechanism when the circuits are being used (i.e. with voltage potentials present) in the presence of moisture.
The tendency for electromigration to occur can be measured by a standard technique specified in Bellcore GR-78-CORE (13.2.5, 13.2.7) standard test procedures which are incorporated herein by reference in their entirety. The foregoing Bellcore procedure measures the average insulation resistance between circuit features. Bellcore and IPC standards require that the average insulation resistance not decrease by more than one decade between the initial value (obtained after a conditioning period of 96 hours at 85xc2x0 C./85% relative humidity with no bias) and the final value (obtained after an additional 500 hours at 85xc2x0 C./85% relative humidity with a 10 V.dc bias applied).
One method which may be used to overcome the electromigration of silver plating is to coat the silver plate with another more noble metal such as gold. The disadvantages of this method are the expense of gold plating as well as the necessity for additional process steps.
Another method which may be used to overcome the electromigration of immersion silver plating is described in U.S. Pat. No. 6,444,109 to Kukanskis, the teachings of which are herein incorporated by reference. In this method, the immersion silver plate is treated with an additive selected from the group consisting of fatty amines, fatty amides, quaternary salts, amphoteric salts, resinous amines, resinous amides, fatty acids, resinous acids, ethoxylated derivatives of any of the foregoing, and mixtures of any of the foregoing.
It is an object of this invention to propose a method for preserving and enhancing the solderability of copper surfaces by plating said copper surface with a novel silver plate which is more resistant to electromigration than prior art silver deposits. It is a further objection of the invention to increase migration resistance of the silver plating as well as to provide tarnish or corrosion resistance of the silver plating by adding a polymer solution to the silver plating bath or by using the polymer solution as a post dip treatment.
The current invention proposes the use of an immersion or electroless silver coating as an improved solderability preservative for various surfaces, particularly copper surfaces. Preferred compositions for depositing the immersion silver coating are also disclosed. The novel silver plating process produces a silver plate, which is more resistant to electromigration than conventional silver deposits. The process proposed is a versatile, low cost method for effectively preserving the solderability of surfaces, particularly copper surfaces and areas of connection on printed circuit boards.
In addition, the current invention proposes to increase the migration resistance as well as the tarnish or corrosion resistance of the silver plating material by using a polymer solution as a post dip on the silver plated surface.